Powysaccharides are often qwite heterogeneous, containing swight modifications of de repeating unit. Depending on de structure, dese macromowecuwes can have distinct properties from deir monosaccharide buiwding bwocks. They may be amorphous or even insowubwe in water.[1] When aww de monosaccharides in a powysaccharide are de same type, de powysaccharide is cawwed a homopowysaccharide or homogwycan, but when more dan one type of monosaccharide is present dey are cawwed heteropowysaccharides or heterogwycans.[2][3]

Naturaw saccharides are generawwy of simpwe carbohydrates cawwed monosaccharides wif generaw formuwa (CH2O)n where n is dree or more. Exampwes of monosaccharides are gwucose, fructose, and gwycerawdehyde.[4] Powysaccharides, meanwhiwe, have a generaw formuwa of Cx(H2O)y where x is usuawwy a warge number between 200 and 2500. When de repeating units in de powymer backbone are six-carbon monosaccharides, as is often de case, de generaw formuwa simpwifies to (C6H10O5)n, where typicawwy 40≤n≤3000.

As a ruwe of dumb, powysaccharides contain more dan ten monosaccharide units, whereas owigosaccharides contain dree to ten monosaccharide units; but de precise cutoff varies somewhat according to convention, uh-hah-hah-hah. Powysaccharides are an important cwass of biowogicaw powymers. Their function in wiving organisms is usuawwy eider structure- or storage-rewated. Starch (a powymer of gwucose) is used as a storage powysaccharide in pwants, being found in de form of bof amywose and de branched amywopectin. In animaws, de structurawwy simiwar gwucose powymer is de more densewy branched gwycogen, sometimes cawwed "animaw starch". Gwycogen's properties awwow it to be metabowized more qwickwy, which suits de active wives of moving animaws.

Nutrition powysaccharides are common sources of energy. Many organisms can easiwy break down starches into gwucose; however, most organisms cannot metabowize cewwuwose or oder powysaccharides wike chitin and arabinoxywans. These carbohydrate types can be metabowized by some bacteria and protists. Ruminants and termites, for exampwe, use microorganisms to process cewwuwose.

Even dough dese compwex powysaccharides are not very digestibwe, dey provide important dietary ewements for humans. Cawwed dietary fiber, dese carbohydrates enhance digestion among oder benefits. The main action of dietary fiber is to change de nature of de contents of de gastrointestinaw tract, and to change how oder nutrients and chemicaws are absorbed.[6][7] Sowubwe fiber binds to biwe acids in de smaww intestine, making dem wess wikewy to enter de body; dis in turn wowers chowesterow wevews in de bwood.[8] Sowubwe fiber awso attenuates de absorption of sugar, reduces sugar response after eating, normawizes bwood wipid wevews and, once fermented in de cowon, produces short-chain fatty acids as byproducts wif wide-ranging physiowogicaw activities (discussion bewow). Awdough insowubwe fiber is associated wif reduced diabetes risk, de mechanism by which dis occurs is unknown, uh-hah-hah-hah.[9]

Not yet formawwy proposed as an essentiaw macronutrient (as of 2005), dietary fiber is neverdewess regarded as important for de diet, wif reguwatory audorities in many devewoped countries recommending increases in fiber intake.[6][7][10][11]

Starch is a gwucose powymer in which gwucopyranose units are bonded by awpha-winkages. It is made up of a mixture of amywose (15–20%) and amywopectin (80–85%). Amywose consists of a winear chain of severaw hundred gwucose mowecuwes and Amywopectin is a branched mowecuwe made of severaw dousand gwucose units (every chain of 24–30 gwucose units is one unit of Amywopectin). Starches are insowubwe in water. They can be digested by breaking de awpha-winkages (gwycosidic bonds). Bof humans and oder animaws have amywases, so dey can digest starches. Potato, rice, wheat, and maize are major sources of starch in de human diet. The formations of starches are de ways dat pwants store gwucose.

Gwycogen is anawogous to starch, a gwucose powymer in pwants, and is sometimes referred to as animaw starch,[13] having a simiwar structure to amywopectin but more extensivewy branched and compact dan starch. Gwycogen is a powymer of α(1→4) gwycosidic bonds winked, wif α(1→6)-winked branches. Gwycogen is found in de form of granuwes in de cytosow/cytopwasm in many ceww types, and pways an important rowe in de gwucose cycwe. Gwycogen forms an energy reserve dat can be qwickwy mobiwized to meet a sudden need for gwucose, but one dat is wess compact and more immediatewy avaiwabwe as an energy reserve dan trigwycerides (wipids).

The structuraw components of pwants are formed primariwy from cewwuwose. Wood is wargewy cewwuwose and wignin, whiwe paper and cotton are nearwy pure cewwuwose. Cewwuwose is a powymer made wif repeated gwucose units bonded togeder by beta-winkages. Humans and many animaws wack an enzyme to break de beta-winkages, so dey do not digest cewwuwose. Certain animaws such as termites can digest cewwuwose, because bacteria possessing de enzyme are present in deir gut. Cewwuwose is insowubwe in water. It does not change cowor when mixed wif iodine. On hydrowysis, it yiewds gwucose. It is de most abundant carbohydrate in nature.

Chitin is one of many naturawwy occurring powymers. It forms a structuraw component of many animaws, such as exoskewetons. Over time it is bio-degradabwe in de naturaw environment. Its breakdown may be catawyzed by enzymes cawwed chitinases, secreted by microorganisms such as bacteria and fungi, and produced by some pwants. Some of dese microorganisms have receptors to simpwe sugars from de decomposition of chitin, uh-hah-hah-hah. If chitin is detected, dey den produce enzymes to digest it by cweaving de gwycosidic bonds in order to convert it to simpwe sugars and ammonia.

Chemicawwy, chitin is cwosewy rewated to chitosan (a more water-sowubwe derivative of chitin). It is awso cwosewy rewated to cewwuwose in dat it is a wong unbranched chain of gwucose derivatives. Bof materiaws contribute structure and strengf, protecting de organism.

Padogenic bacteria commonwy produce a dick, mucous-wike, wayer of powysaccharide. This "capsuwe" cwoaks antigenicproteins on de bacteriaw surface dat wouwd oderwise provoke an immune response and dereby wead to de destruction of de bacteria. Capsuwar powysaccharides are water-sowubwe, commonwy acidic, and have mowecuwar weights on de order of 100–2000 kDa. They are winear and consist of reguwarwy repeating subunits of one to six monosaccharides. There is enormous structuraw diversity; nearwy two hundred different powysaccharides are produced by E. cowi awone. Mixtures of capsuwar powysaccharides, eider conjugated or native are used as vaccines.

Most of dese powysaccharides exhibit usefuw visco-ewastic properties when dissowved in water at very wow wevews.[20] This makes various wiqwids used in everyday wife, such as some foods, wotions, cweaners, and paints, viscous when stationary, but much more free-fwowing when even swight shear is appwied by stirring or shaking, pouring, wiping, or brushing. This property is named pseudopwasticity or shear dinning; de study of such matters is cawwed rheowogy.

Aqweous sowutions of de powysaccharide awone have a curious behavior when stirred: after stirring ceases, de sowution initiawwy continues to swirw due to momentum, den swows to a standstiww due to viscosity and reverses direction briefwy before stopping. This recoiw is due to de ewastic effect of de powysaccharide chains, previouswy stretched in sowution, returning to deir rewaxed state.

Ceww-surface powysaccharides pway diverse rowes in bacteriaw ecowogy and physiowogy. They serve as a barrier between de ceww waww and de environment, mediate host-padogen interactions, and form structuraw components of biofiwms. These powysaccharides are syndesized from nucweotide-activated precursors (cawwed nucweotide sugars) and, in most cases, aww de enzymes necessary for biosyndesis, assembwy and transport of de compweted powymer are encoded by genes organized in dedicated cwusters widin de genome of de organism. Lipopowysaccharide is one of de most important ceww-surface powysaccharides, as it pways a key structuraw rowe in outer membrane integrity, as weww as being an important mediator of host-padogen interactions.

The enzymes dat make de A-band (homopowymeric) and B-band (heteropowymeric) O-antigens have been identified and de metabowic padways defined.[21] The exopowysaccharide awginate is a winear copowymer of β-1,4-winked D-mannuronic acid and L-guwuronic acid residues, and is responsibwe for de mucoid phenotype of wate-stage cystic fibrosis disease. The pew and psw woci are two recentwy discovered gene cwusters dat awso encode exopowysaccharides found to be important for biofiwm formation, uh-hah-hah-hah. Rhamnowipid is a biosurfactant whose production is tightwy reguwated at de transcriptionaw wevew, but de precise rowe dat it pways in disease is not weww understood at present. Protein gwycosywation, particuwarwy of piwin and fwagewwin, became a focus of research by severaw groups from about 2007, and has been shown to be important for adhesion and invasion during bacteriaw infection, uh-hah-hah-hah.[22]